A flash memory device is a kind of a programmable ROM capable of writing, erasing, and reading information. A flash memory device forms a unit string configured of serially connected cell transistors. The memory cells may be the NAND type, which are suitable for high integration since the unit strings are connected between bit lines and ground lines in parallel. The memory cells may be the NOR type, which are suitable for high-speed operations since the cell transistors are connected between bit lines and ground lines in parallel.
Since a NOR type flash memory device can be read at a high speed, it may be used for booting a cellular phone. Since the NAND type flash memory device has a lower read speed but has a faster write speed, it may be suitable for storing data where compactness is a relatively larger consideration.
Flash memory devices may be classified as a stack gate type and a split gate type according to the structure of a unit cell. Flash memory devices may also be classified as a floating gate device and a silicon-oxide-nitride-oxide-silicon (SONOS) device according to the configuration of a charge storing layer. A floating gate device usually includes a floating gate formed of a poly silicon, surrounded by an insulator around it's circumference. To store and erase data, charge is injected into and emitted from the floating gate by a channel hot carrier injection or a follower-Nordheim tunneling.
However, as semiconductor devices tend towards higher integration, smaller design rules must be used for flash memory cells. Accordingly, a 0.13μ flash memory device may have sufficient space for forming a contact in a unit cell. As the size of the unit cell is reduced, the available gap between gate areas forming the unit cell becomes so narrow that a void A is generated after a deposition process for an interlayer dielectric layer as shown in FIG. 1.
The void A changes the characteristics of each cell, creating problems in that the word lines operate differently. To subsequently form a contact, if a metal such as tungsten W, etc., is injected, the tungsten may be diffused toward the void A, causing a contact to contact bridge phenomenon. The tungsten may thereby become bridged with other contacts. The gate formed in the word line may not operate correctly. This causes an error in the cell operation, which significantly degrades the reliability and yield of the flash memory device.